Iris and Rees (second name needs confirmation) are a duo of flying eyeballs. The duo has lower health when compared to other bosses, however, both will split into two small versions of themselves. The smaller versions of the boss appears to have half of the health of its larger flying eyeball cousin. If you apply poison to the larger eye, when it splits the poison will be applied to the smaller eyes.
It is also important to note that each eye you kill will drop one boss worth of loot. This mean you will receive the equivalent of 6 bosses of loot upon successful completion of this boss.